How do I mirror the faces of a cube (edit all of them when I edit one)?
Use one or more mirror modifiers with a mirror object.
Use the array modifier with an object offset.
Use duplifaces displaying another object.


The third option suits your problem best, here is a step by step solution.


Add a plane ad duplicate it Shift + D.





Make sure both planes point to the same mesh datablock. If necessary, set the duplicated datablock to the same mesh. The mesh
datablock in the object properties panel should display a 2 at the
end.




Editing one plane now affects both, because they point to the same datablock.




Parent a plane to the cube Ctrl + P, both should have the same location.





Set the duplication property of the cube to Faces (in the object properties panel).





Editing one of the planes will now affects all faces of the cube. The plane parented to the cube will not be rendered. I duplicated the plane to make the edits to it easier, since the other plane is hidden in the cube.